Laser light has been used to initiate the ignition of fuel/oxidizer mixtures for more than a decade. Recent developments have included laser induced ignition of liquid fuel aerosols to overcome problems with capacitive discharge igniters. State of the art laser-based ignition processes use a laser-spark, air-breakdown ignition method in which a single, high peak-power, short duration laser light pulse is used to initiate fuel ignition via the generation of a high temperature, air-breakdown, ionization plasma. The performance of this spark breakdown ignition method to reliably ignite fuel aerosols is limited to a narrow range of fuel parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel and air temperatures.
Laser spark breakdown ignition of fuel/oxidizer mixtures occurs basically in four steps: (1) non-resonant multiphoton ionization of gas molecules generating a light absorbing plasma via electron cascade; (2) deposition of thermal energy and vaporization of fuel droplets; (3) initiation of combustion through both thermal and photo-chemical reaction of fuel and oxidizer; and (4) formation and propagation of the flame kernel to regions outside the initial site of plasma formation.
The plasma formation of step (1) requires the application of high pulse energy and high peak power density laser light. This requirement necessitates the use of a large-sized laser source and short duration laser pulses which are no more than tens of nanoseconds in pulse length. High peak laser power can cause the formation of intense shock waves within the ignited fuel which can cause self-extinguishing of the laser induced ignition flame.
Typically, a Q-switched laser with a pulse width and pulse energy which will provide the high peak power density required to initiate plasma formation is used to initiate plasma formation and satisfy concurrently the need for time-averaged power for sustaining ignition.
Therefore, there is a need for an energy efficient process for initiating and sustaining the ignition of a broad range of aerosol fuel/oxidizer mixtures.
There is also a need for a laser ignition process which can reliably ignite aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures.
Economical improvements in ignition technology are also needed.
Therefore, is an object of this invention to provide a method for improved ignition performance.
It is another object of this invention to provide a method for initiating and sustaining ignition of aerosol fuel.
It is yet another object of this invention to provide a laser ignition method having reduced peak power requirements.
It is a further object of this invention to provide a method of laser ignition which can use a smaller, less complex laser.
It is still another object of this invention to provide a fuel pre-heat method for enhancing the stability of fuel combustion.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.